Source: The Perils of the top of doubleton lead
Do you like leading from a doubleton (vs a suit contract)? Generally speaking, I do not.
I will almost never lead from an honour-doubleton (e.g. Q4, J8). When I lead the honour [and unless you live in Poland, it is universally standard to lead top from two cards], my partner will think I have the card immediately beneath (and am leading top of a sequence). Furthermore, the honour is too valuable merely to toss on the table and (in all likelihood) lose.
I will rarely lead (top) from a small doubleton either. Unless partner has at least five cards in that suit (in which case you might hear an overcall – a totally different situation in which I am now happy to lead the doubleton), then either declarer or dummy must have at least four cards. I am helping to set up their suit, and exposing my partner to finesses in the process.
What happened
West led 
7, top from his doubleton. This gave declarer all four diamond tricks, by finessing East out of his guarded J. Trick One went 7, 4, J, A, whereupon declarer drew trumps and played 10 and over to KQ, discarding 
4. He now had the luxury of leading towards his 
K for the overtrick. This did not materialise with West holding A over K, but the game was made.
What should have happened
West immediately eliminates a club lead (leading away from an ace, against a suit contract, is bottom of the heap; here you can see that it promotes declarer’s K). But he prefers a heart to the top of a doubleton diamond. 3 goes to 2, 10 (note that there is no reason for East to play K, as he knows West cannot hold A; indeed it is imperative that he keeps K, to prevent dummy’s J from being promoted). Declarer wins A, cashes 
AK, then follows with A and over to KQ. J does not appear and now, with just three diamond tricks, declarer is left leading to K for his game. When K loses to A, he is down one, losing two clubs and two hearts.
If you remember one thing…
Beware the (top of a) doubleton lead. It often helps declarer to set up a side-suit.